Cellulose pulp is bleached for a number of reasons. One reason is to decrease the lignin content of the pulp, whereas in the case, for example, of chemical pulps the objective is to remove the lignin totally. Another reason for bleaching pulp is to increase its brightness. It is often endeavoured to both remove lignin from the pulp and to increase the brightness thereof.
Different bleaching agents have different effects on cellulose pulp, and can be divided into bleaching agents which remove lignin and bleaching agents which preserve the lignin. Examples of lignin removing bleaching agents include chlorine, chlorine dioxide, hypochlorite, chlorite, oxygen gas and ozone. Various kinds of peroxide, dithionite and boron hydride constitute examples of lignin preserving bleaching agents. Some of these bleaching agents possess both of the aforesaid properties. The pulp is normally treated with alkali between the various bleaching stages and washed thereafter to remove dissolved material.
A number of parameters must be taken into account when using a given bleaching agent, in order to achieve the result desired. For example, it is normally necessary to check or monitor pulp consistency, temperature, time and pH, and also preferably to control these parameters. Another important parameter is the bleaching agent content, partly of the actual bleaching liquor as such and partly subsequent to mixing the bleaching liquor with another liquid phase in the absence or presence of the pulp. There are many theories as to how a bleaching stage should be controlled. Depending upon the method of control applied, the bleaching agent content is determined immediately after mixing the agent with the pulp, at some time during the bleaching stage, or upon completion of the bleaching process. The bleaching agent is often charged to the system in excessive quantities, in which case the amount of bleaching agent that remains is normally of interest.
A relatively common method of controlling the supply of bleaching agent to the system, and therewith the amount of bleaching agent contained by the pulp suspension is one of measuring the redox potential of the suspension and of regulating the charge of bleaching agent going out from a predetermined set-point value of the redox potential, so that the set-point value is reached. In the case of bleaching chemicals, this method can only be used within a very narrow content range. In addition this particular method of measuring the redox potential in the pulp suspension is dependent on temperature, to which is added the problem of coatings on the measuring electrodes.
From the childhood of bleaching technology up to the present day, samples of bleaching agent solutions have been taken manually (and are still taken manually) at relatively wide time intervals, these samples being titrated manually, e.g. iodometrically, to arrive at a given bleaching agent content.
The samples are taken at one or more locations within the bleaching department, e.g. from the actual bleaching liquor before it is mixed into the pulp, or from the suspension liquor subsequent to mixing the bleaching liquor into the pulp, i.e. during an ongoing bleaching process, or from the suspension liquid subsequent to completion of a bleaching process, i.e. in order to determine the residual content of bleaching chemicals of the suspension liquor. This particular methodology results in poor coverage of what actually takes place during bleaching of the pulp, partly because the samples are taken at random, and partly because of the relatively long period of time that lapses from the time of taking the sample to the time at which the operator in the bleaching department receives the information concerning the content of the bleaching agent concerned.
With regard to strongly oxidative bleaching agents, such as chlorine and chlorine dioxide, it has been suggested in Swedish Patent Specification No. 7314129-3 (399 966) that samples of liquor containing these bleaching agents shall be reacted with a chemiluminescent reactant, leading to the emission of light. The total amount of light emitted is measured, and the value obtained provides data relating to the content of the bleaching agent concerned. The method in question is intended primarily for determining both the chlorine content and the chlorine dioxide content of one and the same bleaching liquor sample, which has previously been impossible to achieve to any satisfactory degree of success. This method can be applied to particular advantage in bleaching stages in which a mixture of chlorine and chlorine dioxide is used.